Forensics Lab 6.11: Make Transverse Sections of Fiber Specimens

In the preceding lab sessions, we’ve observed hairs and fibers longitudinally. As useful as a longitudinal view is, it doesn’t reveal all of the visible information about a specimen. To complete our microscopic examination, we need to prepare and view a transverse section (also called a cross-section) of the specimen.

Because the surface of a specimen is seldom perfectly transparent, a transverse section may reveal details about the internal structure of a specimen that are obscured in a longitudinal view. For example, it may be obvious from a longitudinal view that a fiber has been dyed, but the “end-on” view of a transverse section allows us to determine whether the dye is spread uniformly throughout the fiber or concentrated near the surface. Similarly, it can be difficult to determine the cross-sectional shape of a fiber from a longitudinal view. With a transverse section, it’s immediately obvious if the cross-section is circular, oval, oblate, kidney-shaped, triangular, lobed, irregular, or otherwise.

Producing usable transverse sections in a home lab requires some effort. It’s easy enough, of course, to cut a hair or fiber. The hard part is getting a thin transverse section mounted and correctly oriented on a slide. Several means can be used to obtain transverse sections of hairs and fibers, including the following:

Cross-section test slide method

Professional forensics labs generally use special metal microscope slides designed for sectioning hair and fiber specimens. These slides are made of 0.25 mm thick stainless steel, punctured with numerous 0.9 mm holes. In use, a fiber bundle is drawn through a hole using a short length of thin, soft copper wire (about 34 AWG) to pull the looped fiber bundle through the hole. If necessary, the fiber bundle can be bulked out to fill the hole by using known fibers. Using a scalpel, craft knife, or single-edge razor blade, the fiber bundle is sliced off flush with the top and bottom surfaces of the plate, leaving 0.25 mm thick fiber sections that can be viewed by reflected or transmitted light.

Plastic slide method

You can accomplish much the same thing with inexpensive materials just by drilling several small holes in a plastic slide using a drill and small bit, as shown in Figure 6-25. Bits from about #61 (0.039″ or 0.991 mm) through about #75 (0.021″ or 0.533 mm) make holes of useful sizes for fibers of differing thicknesses. If you don’t have a drill, you can use a heated needle to melt holes in the slide, but plan to waste one slide for practice before you master melting holes that don’t have excessively raised edges.

Once you have a slide with usable holes, simply draw fiber bundles through each hole using a needle or thin copper wire and then use a scalpel to slice off the excess fiber flush with both surfaces of the slide. Depending on the thickness of the plastic slide, this method yields thicker sections, typically 0.5 mm to 1.0 mm, which may be too thick to view by transmitted light.

Figure 6-25. Drilling a plastic microscope slide for use as a sectioning slide

Drillzilla

We didn’t have a hobby drill when we wrote this chapter, so we decided to see how our regular 3/8″ DeWalt corded drill would work. We used the smallest bit we had available (1/16″ or 1.5875 mm), oriented the drill as precisely vertical as we could, and drilled away. Surprisingly, the resulting hole was smooth and circular. We initially thought it would be too large, but as it turns out the larger hole makes it easier to obtain cross sections. We simply looped a batch of fibers, pulled them through the hole with a needle until they were wedged tightly, and then cut them off flush with a razor knife.

Using cross-section slides is faster if you have several specimens to section, but you can also section specimens individually using one of the following methods.

Commercial microtome

Lab supply vendors sell commercial microtomes and casting medium suitable for making thin sections. Student-grade microtomes are available for $35 or so, and are precise enough to produce sections as thin or thinner than those produced with cross-section slides.

Home-made microtome

You can make a microtome using a fine-thread bolt and a matching flat nut. To use this method, thread the bolt one or two turns into the nut, leaving the remaining part of the nut to form a well. Using forceps, hold the fiber vertically in that well as you drip molten candle wax into the well, filling it slightly overfull. Allow the wax to solidify and then use a scalpel, craft knife, or single-edge razor blade to trim off the excess wax flush with the face of the nut. Discard this first section, and then turn the nut slightly to drive a thin section above the face of the nut. Use the scalpel to slice off that thin section and mount it on a slide. Used with care, this method can produce sections of 0.25 mm or thinner.

Drinking straw method

The drinking straw method is simple, requires only items found around the house, and can give good results. It is also fussy, requires an assistant for best results, and may require multiple attempts to get a good section. Cut a short (~ 5 cm) length of plastic drinking straw. Lubricate the inside surface with a very small amount of oil, WD-40, or similar lubricant. Place the straw section vertically against a safe work surface, using the bottom edge of the straw to pin one end of the fiber against the work surface. Using forceps, hold the other end of the fiber as nearly vertical as possible centered in the straw and have your assistant drip candle wax into the straw until it is nearly full, being careful not to burn the fiber or yourself. Allow the wax to harden for a minute or so, and then use a dowel or similar object to slide the wax plug until the end is just past flush with the end of the straw. Use the scalpel to trim off the first section and discard it. Press the plug a bit further out of the straw and carefully use the scalpel to cut a section as thin as possible.

Dripping wax method

One of the easiest ways we found to make a fiber cross section is to drip melted candle wax onto a microscope slide until you have a mound built up about 3mm thick. Carefully embed the fiber into the wax before it hardens completely. Then drip more wax onto the fiber to build up the mound another 3mm or so, as shown in Figure 6-26. Once the mound hardens completely use a scalpel or razor knife to cut thin vertical sections through the wax.

Figure 6-26. Making a fiber cross section with melted candle wax

Cork method

We include this method for completeness, although we haven’t had much luck with it. Begin with a high-quality, fine-grain cork section about 2.5 cm thick. Press a thin needle through the cork, using pliers if necessary, until the eye of the needle is just visible above the surface of the cork. Thread your fiber specimen through the eye of the needle, and draw the needle all the way through the cork, stopping before you pull the fiber specimen completely free of the cork. Use a scalpel to cut the fiber free from the needle, flush with the surface of the cork. Then use the scalpel to cut a thin section of the cork with the fiber specimen embedded. The problem we had was that the cork crumbled when we tried to cut thin sections. Perhaps our corks just weren’t good enough.

In this lab session, we’ll make cross sections of our known and questioned fiber specimens and examine those sections microscopically to learn what we can see from the cross-sectional perspective that is invisible with longitudinal whole-mount specimens.

Required Equipment and Supplies

goggles, gloves, and protective clothing

compound microscope (100X and 400X, with ocular micrometer)

equipment for sectioning (See Introduction)

known and questioned fiber specimens

The Maker Shed carries a variety of chemistry supplies and equipment. Check out the Science Room section to see what’s currently available.

CAUTIONS

Although none of the activities in this lab session present any significant risks, other than perhaps cutting yourself while making sections, as a matter of good practice you should always wear splash goggles, gloves, and protective clothing when working in the lab, if only to avoid contaminating specimens. Obviously, you may need to remove your goggles while using the microscope.

Procedure

If you have not done so already, put on your goggles, gloves, and protective clothing.

Make and observe cross sections for as many known and questioned fibers as you have time for. Make and label sketches of each cross section in your lab notebook, or shoot images of each cross section and attach labeled copies of each image in your lab notebook. If time allows, make and observe cross sections of your various human and animal hair specimens as well.

Review Questions

Q1: What advantages do fiber cross sections have relative to longitudinal sections?